Plain Language Summary
Magma chambers in the continental crust are believed to be “mushy”, meaning they are reservoirs rich in both crystals and magma. The magma occupies the pore space between a connected network of crystals and the difference in density between crystal and magma leads to separation. During the separation process, the crystal networks behave like a sponge and magma percolates upward and is extracted as the pore space in the network of crystals closes. Magma collects atop the “sponge” and can potentially go on to feed volcanic eruptions at the Earth’s surface. Therefore, how quickly it can separate has implications for monitoring volcanic hazards. When the porosity of the “sponge” is sufficiently low, the closing of pore space can only proceed if individual crystals are deformed (bent, for instance) and the process is slow. At larger porosities, however, the pore space can be closed by the sliding or rotation of crystals. We model this sliding process and compare our model to analog experiments and find that sliding may allow for this process to be efficient.
1 Introduction
Forecasting volcanic hazards depends largely on our understanding of how quickly melt can mobilize and congregate. The physical mechanisms that lead to phase separation in silicic, crustal magma systems span a spectrum of melt fractions, ranging from crystal settling at large melt fractions to compaction (closure of pore space in matrix forming crystals) at intermediate to low melt fractions (< ca. 0.6 – 0.7) (Bachmann & Bergantz, 2004, Bachmann & Huber, 2019, Holnesset al. , 2017, McKenzie, 1984, Richter & McKenzie, 1984). The extent to which melt can be effectively separated and the associated timescale of this process depends on the physical mechanism operating.
Several experimental datasets have been presented by researchers that aim to constrain the rheology of multiphase magma systems. These datasets include deformation experiments on partially molten rock samples, typically exploring melt fractions ranging from ca. 0 – 0.3 (Hirth & Kohlstedt, 1995, Lejeune & Richet, 1995, Mei et al. , 2002, Renner et al. , 2003, Scott & Kohlstedt, 2006) and can be used to explore phase separation mechanisms. This suite of experiments subject partially molten rock samples to applied stresses or controlled pressure differences and record strain rates as a function of melt fraction. A combination of stress-strain rate and microstructure analysis provides insight into the mechanism by which the experimental samples deform. Over the past two decades, a suite of centrifuge experiments has also been developed, which can be used to constrain the rheology of multiphase magma systems (Bagdassarov et al. , 2009, Connolly et al. , 2009, Krättli & Schmidt, 2021, Manoochehri & Schmidt, 2014, Schmidt et al. , 2012). One group of these centrifuge experiments (Connolly et al. , 2009) have been conducted at melt fractions < 0.3, while another group (Schmidt et al. , 2012) were conducted at intermediate melt fractions (ca. 0.3 – 0.7). Connolly and Schmidt (2022) inferred that the centrifuge experiments were limited by grain boundary-controlled diffusion (GBD). Microstructural evidence for GBD was reported in this suite of experiments and included flattened, melt-free grain contacts. Furthermore, the viscosities inferred by analysis of both sets of experiments are self-consistent and imply compaction rates much higher than determined from the suite of partially molten rock experiments (Fig. 1 ).